A number of techniques can be used to complete a well and prepare it for production. For example, a borehole may have casing cemented therein. To prepare the borehole for production, operators perform a plug and perforation operation. To do this, a jetting tool and a milling tool are run on coil tubing into the cemented casing to clean out residual cement. The jetting tool is then used to initially perforate the casing at the toe of the borehole.
Once these initial perforations are formed, a wireline-deployed perforating gun and a bridge plug are pumped down the casing. The bridge plug is set in the casing to isolate the lower zone of the borehole, and the perforating gun perforates the casing. The wireline is removed from the borehole, and fracture treatment is pumped down the casing to fracture the zone at the perforations in the casing. This operation of pumping down a plug, perforating the casing, and pumping fracture treatment is then repeated multiple times up the borehole until a desired number of zones in the formation have been fractured. In final stages, the bridge plugs can be milled out of the casing using a milling tool.
Although such operations may successfully prepare a well for production, there may be a need at some point in the life of the well to re-fracture the existing borehole even though the borehole was originally completed and hydraulically fractured using the plug and perforation operation. To perform the re-fracture operation, a traditional zonal pressure isolation system, generally consisting of smaller tubing mounted with packers and fracture sleeves, can be inserted into the existing casing so various zones can be re-fractured.
For example, FIG. 1 shows a wellbore 10 having cemented casing 12 and perforations 14. This wellbore 10 may have been initially fractured using plug and perforation operations. To re-fracture zones, a wellbore system 20 having an inner tubing string 22 is deployed in the casing 12 from a rig 24. The tubing string 22 has various sliding sleeves 30 and packers 40 disposed along its length at particular zones to be re-fractured. The packers 40 are set inside the casing 12 to isolate the wellbore annulus 16 into isolated zones.
The sliding sleeves 30 deployed on the tubing string 22 between the packers 40 can be used to divert treatment fluid to the isolated zones of the surrounding formation through the casing's perforations 14. As conventionally done, operators rig up fracturing surface equipment 26 for pumping fluid down the tubing string 22. In stages of operation, operators then deploy specifically sized balls to open the sliding sleeves 30 between the packers 40 and to divert fracture treatment to each of the isolated zones up the wellbore 10.
Historically, the packers 40 used for zonal isolation in the re-fracture operations have elastomeric packing elements, such as swellable elements, cup packers, or hydraulically compressed packing elements. As can be seen, such a traditional isolation system 20 has a restricted inner dimension because the tubing string 22 must have a dimension capable of fitting in the casing 12. Additionally, the tubing string 22 must be dimensioned so that the sliding sleeves 30 and the packers 40 deployed on the string 22 can operate properly in the available annulus 16 between the tubing string 22 and the existing casing 12. The restricted inner dimension of the tubing string 22 caused by these requirements may make the system 20 unacceptable for use at high fracture injection rates.
One alternative way to perform a re-fracture operation can use a larger internal tubing string that installs in the existing casing 12. This larger tubing string allows a secondary plug and perforation operation to be performed in the wellbore 10. As expected, the annular space between the outer dimension of such a larger internal string and the inner dimension of the existing casing 12 is very limited, and this limited dimension makes isolating the zones along the borehole difficult to achieve. In fact, there may be insufficient room to create a suitable seal between the tubing string and the casing 12 that the objective of zonal isolation cannot be achieved for the new plug and perforation operation. The small annular gap might be an application where swellable elastomers could be used. However, there may be no activation fluid available in low fluid level wells for the swellable elastomer to function properly.
Another alternative way to perform a re-fracture operation can use a large diameter tubing string inserted into the existing casing 12 to tightly fit in the casing 12. It is believed that the tight fit between the inner and outer strings diverts the fracture treatment fluid albeit without a seal. One other solution includes mechanically deforming a tubular against the inner dimension of the casing 12 to create the desired zonal isolation, but such systems are very expensive and difficult to implement. Lastly, chemical/cement squeezes have been used for re-fracture operations, but these methods tend be unsatisfactory for pressure integrity and are likewise expensive.
The subject matter of the present disclosure is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.